The speedometer needle has started bouncing around and there’s a strange humming noise that gets louder the faster I go. What do you think it could be?
What vehicle? 1986 Toyota Pickup, (4wd, 5-speed, Base model)
Does it get worse when it’s cold, or get better when the truck warms up? Yes it gets worse on cold mornings. Sometimes it goes away completely when the truck is fully warm on a sunny day.
Diagnosis by phone: This truck has a cable driven speedometer (as opposed to an electric) and so this diagnosis is almost 100% sure thing. Typically the cable gets dry in the sheath and then starts to bind on the curves. This creates the noise. The bouncing motion is because the cable spins a little and then slows down and then spins some more. A test drive will confirm that the sound is indeed coming directly from behind the gauge cluster.
Final diagnosis: After the test drive it is confirmed that the speedo cable does need to be replaced.
Solution: Replace the speedo cable. Once this has been done we take another test drive and now have a quiet cabin. The speedometer sits solid on the correct speed.